The assessment and therapy of autoimmune diseases are poorly studied. This is in part due to the rarity and heterogeneity of many of these disorders as well as the uncertainty of their pathogeneses. We are focusing on the idiopathic inflammatory myopathies (IIM) as prototypic autoimmune diseases from which we hope that principles learned in these disorders may be applied generally to other autoimmune diseases. The IIM are orphan diseases, with an estimated incidence of only 10 cases per million per year, but yet they are associated with substantial lifelong morbidity and mortality. Their treatment is unsatisfactory in many cases and drug efficacy evaluation is hampered by the lack of reliable and standardized measures of disease outcome. Consequently, drug safety and efficacy data tend to be from small-uncontrolled non-comparable clinical trials. No agents are currently licensed for the treatment of the IIM and few have been studied in randomized controlled clinical trials. Because of the lack of consensus on how to best measure disease in the myositis syndromes, and to enhance recruitment of patients for other studies, new disease assessment tools are being developed and validated to apply to all forms of myositis in both adults and children. We have organized a collaborative group of over 100 adult and pediatric specialists with special interest in myositis, known as the International Myositis Assessment and Clinical Studies Group (IMACS) to assist in this project. The aims of this project are to develop and validate sensitive and efficient disease activity and damage measurement tools, define improvement criteria for use in clinical trials, and develop consensus on a number of clinical trial design parameters. Achieving these goals should enhance the consistency by which clinical trials are performed, improve the capacity to compare different treatments, and encourage development of promising novel therapeutic agents. Therapeutic studies of novel biologic agents are in progress and others are being planned to utilize these outcome measures in prospective trials.